Conventionally, as a first prior art articulated robot there has been one that is shown in FIG. 5. This robot comprises a base 2 rotating on a base plate 1 about a vertical rotation axis (S-axis) by means of an S-axis drive motor 21 provided on the base plate 1; a first arm 3 rotating on the base 2 about a horizontal rotation axis (L-axis) by means of an L-axis drive motor 31 provided on the base 2 and capable of tilting forward and backward; a second arm 4 rotating at the top end of the first arm 3 about a rotation axis (U-axis) parallel to the L-axis by means of a U-axis drive motor 41 provided on the base 2; a third arm 5 rotating about a rotation axis (R-axis) extending in the longitudinal direction of the second arm 4 by means of a R-axis drive motor 51 provided on the second arm 4; a wrist portion 6 rotating about a vertical rotation axis (B-axis) perpendicular to the longitudinal direction of the third arm 5; a rotary portion 7 attached to the wrist portion 6 and rotated about a rotation axis (T-axis) perpendicular to the B-axis by means of a T-axis drive motor 71 provided on the second arm 4; and a workpiece clamp portion 81 attached to the rotary portion 7 so as to clamp a workpiece W. The second arm 4 or the third arm 5 of the articulated robot are to be especially long to expand the operating range of the robot.
Further, the R-axis, B-axis and T-axis drive motors 51, 61 and 71 are provided at the root portion of the second arm 4, and the second and third arms 4 and 5 are made hollow so that drive shafts 52, 62 and 72 can be passed through these hollowed arms to thereby rotate the third arm 5, the wrist portion 6 and the rotary portion 7. (e.g., Japanese Laid-Open Patent Publications H1-257590 and H2-41888).
In addition, in order to keep the attitude of a workpiece constant during transfer, there is a second prior art articulated robot as shown in FIG. 6. This robot is constructed so that a horizontally rotatable arm 76 is provided at an articulated portion 75, a parallelogrammatic link mechanism 77 is attached to the arm 76 which makes itself one side of the parallelogram formed by the mechanism, and a wrist horizontal swing mechanism is attached to the top end of the link mechanism 77 to allow a wrist portion 73 to swing horizontally in a way that the attitude of the wrist portion 73 is maintained by rotating the parallelogrammatic link mechanism 77 in the vertical direction. (e.g., Japanese Laid-Open Patent Publication No. 59-146774).
However, the first prior art articulated robot the has the problem that, when the second arm 4 or the third arm 5 is elongated, the drive shafts 52, 62 and 72 for rotating the wrist portion 6 and the rotary portion 7 are made longer to cause the reduction of the resonance frequency of the driving shaft resulting in a limitation to the critical number of revolutions, so cannot operate at a high speed. Further, there has also been the problem that the moment applied on the second arm 4 or the third arm 5 increases, making it necessary to increase the strength of the second arm 4 or the third arm 5 when the clamp device 8 attached to the wrist portion 6 or clamped to workpiece W interferes with any other substance. Consequently, it has not been possible to make the second arm 4 or the third arm 3 especially long.
Further, in order to keep the attitude of the workpiece constant during transfer, one may have the idea of mounting the arm 76 of the second prior art articulated robot horizontally to the wrist portion 6 of the first prior art articulated robot. But since the parallelogrammatic link mechanism 77 of the second prior art must be rotated, the drive section has to be pivotally supported which results in reducing the rigidity of the connected sections and causes the necessity of providing a heavy-weight motor for the wrist swing mechanism for swinging the wrist portion 73.
In the first prior art articulated robot and also in the combination of the first and second prior art articulated robots, the clamp device 8 having an elongated shank 82 extending from the wrist portion 6 up to the work clamp portion 81, or having the parallelogrammatic link mechanism 77 is attached to the wrist portion 6 as shown in FIG. 4(b) and, for example, when a rectangular workpiece W is clamped by the clamp device 8 and moved from a point P to a point Q, the inertia about the T-axis will be expressed by: 4.times.M.times.L2+M (a2+b2)/3; here, M designates the weight of the workpiece, L designates the distance from the center of the wrist portion 6 to the center of gravity of the workpiece W and a, b designate the lengths of the two sides of the workpiece, respectively.
Accordingly, the inertia about the T-axis increases as the length of the clamp device increases so that the loads applied on the T-axis drive motor 71 for the wrist portion 6 and the rotary portion 7 also increase resulting in reducing the life of the speed reducer connected to the drive motor 71. Therefore, the present invention's object is to provide a wrist mechanism for an articulated robot in which only a small loads are applied on each of the drive motors and with which the robot can be operated at a high speed without reinforcing its arms.